The symptom
The MacBook suffered liquid damage. Water was spilled near the trackpad. The first thing to do is to shut the Mac down completely and bring it in for repair as quickly as possible so the battery can be disconnected to minimize damage.
Unfortunately, this Mac is a good example of time working against you…
The diagnosis
A visual inspection was carried out to assess the damage from the water ingress. Unfortunately, as soon as the battery was disconnected, there was a nasty surprise: the BMS connector was completely burnt and had burned a hole through the board.
The daughterboard was also affected.
What was done
First, the motherboard was removed for a more thorough diagnosis and cleaning of the oxidized areas. This got the board working again, but the burnt BMS connector had created a short circuit on the PPVBAT_AON line, the battery line.
This doesn't prevent the board from booting, but it does make it unrepairable. Indeed, the short circuit is located inside the board itself. A repair attempt was made, but the short circuit persisted regardless, because the carbon produced by the burn is conductive and shorts out the different traces making up the motherboard.
A bypass line was created via a jump wire, isolating the shorted line on the board, but it didn't work.
Result
Unfortunately, the repair was not a success: since the liquid damage had occurred 4 months earlier, oxidation had spread too far; the BMS connector had burned violently, creating an internal short circuit that is rarely repairable.
This highlights the importance of bringing a device that has suffered liquid damage to your repair shop as quickly as possible.